IDH1 and neoplasm: The subsequent gain of TP53 mutations or loss of 1p19q further downstream establishes astrocytic and oligodendroglial phenotypes, respectively.8 The most common mutation occurs at a single amino acid residue of IDH1, arginine 132, which is commonly mutated to histidine (R132H).9 The IDH2 mutations, although less common than IDH1 mutations, occur at arginine 172 (R172).10 The IDH1 and IDH2 mutations are mutually exclusive, in that a tumor will only have one of these two mutations.